Abstract: Pre-cooled combined-cycle engine shows a broad application perspective in aerospace field due to its superiority of wide-speed-spectrum, high specific impulse and thrust-to-weight ratio. First of all, this paper reviews the operation principle, technology characteristic and development progress of some typical pre-cooled combined-cycle engines, such as LACE, SABRE and ATREX. Analyses on critical thermal problems were carried out as well. Secondly, numerical models aimed at pre-cooler, compressor, turbine and combustor, etc. were established to investigate the fuel requirement of pre-cooling and combustion, the parameters matching method of pre-cooler and compressor, the synergetic working mode of compressor and turbine. The results shows that hydrogen with 1.0～2.0 times the equivalence ratio can cool the air by 51～476 K at Ma 0～4.5, while methane with the same mass flowrate can only cool the air by 24～182 K at Ma 0～4.0. The entropy function is applied to characterize the total energy loss of pre-cooler and compressor due to the thermal potential difference effect and the work-heat conversion. Different cooperative operation lines of equivalence ratio and pressure ratio ( \varphi - \pi _\textc ) can be designed on this entropy function diagram to meet the engine performance demands. The total power of turbine is the main reason affecting the compressor pressure ratio of pre-cooled engines. The massflow rate of working medium (coolant) for the pre-cooling engine turbine is relatively less compared with traditional turbine, which brings greater challenges to turbine design. Finally, taking the evaluation results into account, some recommendations for future research focuses on pre-cooled combined-cycle engine are proposed.